Harvest constitutes a major
operation among agricultural activities. Considered for a long
time as the last step in production, it must rather be approached
as the first one in the postproduction system, because of its
influence on subsequent processing and preservation of the
products.

Harvesting methods differ
according to the part of the plant to be used. As regards forage
crops, the whole plant is cut, but for underground crops (eg,
groundnuts, roots and tubers), the crop is lifted while the soil
sticking to it is removed. With cereals, the crop is first cut
either as a whole or partially (ears), and then threshed and
cleaned to separate the grain from the ears and straw.

In the latter case two main
alternatives exist: separate harvesting and threshing, or
combined harvesting and threshing.

In developing countries the first
alternative is generally the most widely applied. Although
harvesting and threshing are still frequently done by hand, their
mechanization has begun to develop during recent years,
especially where the crop is produced not for self-consumption
but rather for commercial purpose. Nevertheless, such
mechanization has not developed everywhere to the same extent but
according to the type of crop concerned, because labour
requirements remain high for handling the produce before
threshing.

In industrialized countries,
attempts have been made since the beginning of the 20th century
to devise machines which would both harvest and thresh grain, so
as to reduce the labour requirements involved. Combine harvesters
('combines') which can cut, convey, separate and thresh the grain
were the product of this development work. They are in widespread
use, and have been used already on large grain production schemes
in a number of developing countries.

Rice Harvesting and Threshing

(i) Harvesting methods

Manual harvesting

In many countries rice ears are
cut by hand. A special knife is frequently used in SouthEast Asia
("ani-ani"), Latin America ("cuchillo") and
Africa. For instance, in the Casamance region of Senegal rice is
cut stem by stem with a knife, 10 cm below the panicle so as to
leave straw in the field in amounts large enough to produce
grazing for cattle. Nevertheless such practice is labour
intensive.

To harvest denser varieties (500
stems/sq metre instead of 100) a sickle is used mainly on a
generally wetter produce. But work times remain high: 100 to 200
man-hours per ha for cutting and stooking.

Mechanized harvesting

During past decades the
mechanization of rice harvesting has rapidly evolved. It first
developed in Japan, then in Europe and has now reached many
tropical countries.

The first machines used were
simple animal-drawn (horses in Europe, oxen in the tropics) or
tractor-driven mowing machines fitted with a cutter bar. The
improvements made on this equipment have first resulted in the
development of swathers (Figure 4.1. Swather.). These drop the crop in a continuous
windrow to the side of the machine making it easy to pick up the
panicles and manually tie them into bundles. The next step
forward has been the reaper that forms unbound sheaves; and
finally the reaper/binder which has a tying device to produce
sheaves bound with a twine. However the supply, cost and quality
of the twine are the main problems associated with the use of
such equipment.

The output of these machines
varies between 4 and 10 hours per hectare, which is slow.
However, they may be usefully introduced into tropical rice
growing areas, where hand harvesting results in great labour
problems. In temperate countries they have been gradually
replaced by combine harvesters.

(ii) Threshing methods

After being harvested paddy
bunches may be stacked on the plot. This in-field storage method
results in a pre-drying of the rice ears before threshing, the
purpose of which is to separate seeds from panicles.

Traditional threshing

The traditional threshing of rice
is generally made by hand: bunches of panicles are beaten against
a hard element (eg, a wooden bar, bamboo table or stone) or with
a flail. The outputs are 10g to 30kg of grain per man-hour
according to the variety of rice and the method applied. Grain
losses amount to 1-2%, or up to 4% when threshing is performed
excessively late; some unthreshed grains can also be lost around
the threshing area.

In many countries in Asia and
Africa, and in Madagascar, the crop is threshed by being trodden
underfoot (by humans or animals); the output is 30kg to 50kg of
grain per manhour. The same method, but using a vehicle (tractor
or lorry) is also commonly applied. The vehicle is driven in
circles over the paddy bunches as these are thrown on to the
threshing area (15m to 20m in diameter around the stack). The
output is a few hundred kg per hour. This method results in some
losses due to the grain being broken or buried in the earth.

In south-east Asia, total losses
induced by traditional harvesting and threshing methods are
estimated between 5 and 15%.

Mechanized threshing

From a historical viewpoint,
threshing operations were mechanized earlier than harvesting
methods, and were studied throughout the 18th century.

Two main types of stationary
threshing machines have been developed.

The machines of Western design
are known as 'through-flow' threshers because stalks and ears
pass through the machine. They consist of a threshing device with
pegs, teeth or loops, and (in more complex models) a
cleaning-winnowing mechanism based upon shakers, sieves and
centrifugal fan (Figure 4.2. 'Through-flow' Thresher.). The capacities of the models from
European manufacturers (eg, Alvan Blanch, Vicon, Borga) or
tropical countries (Brazil, India, etc.) range from 500 to 2000kg
per hour.

In the 70s, IRRI developed an
axial flow thresher which has been widely manufactured at local
level. Such is the case in Thailand where several thousands of
these units have been put into use. They are generally mounted on
lorries and belong to contractors working about 500 hours per
year.

More recently, a Dutch company
(Votex) has developed a small mobile thresher provided with
either one or two threshers (Figure 4.3). The machine has been
widely adopted in many rice growing areas. The simple design and
work rates of these machines (about 500kg per hour) seem to meet
the requirements of rural communities.

The 'hold-on' thresher of
Japanese design (Figure 4.4. 'Hold on' thresher - Japanese
design.), is so-called
because the bundles are held by a chain conveyor which carries
them and presents only the panicles to the threshing cylinder,
keeping the straw out. According to the condition of the crop,
work rates can range between 300kg and 700kg per hour (Iseki
model). The main disadvantage of these machines is their
fragility.

(iii) Combined harvesting and threshing methods

Combine-harvesters, as the name
implies, combine the actions of reaping and threshing. Either the
'through-flow' or the 'hold-on' principle of threshing may be
employed, but the reaping action is basically the same. The main
difference is that combine-harvesters of the Western
('through-flow') type are equipped with a wide cutting bar (4-5m)
while the working width of the Japanese ('hold-on') units is
small (1m). According to the type of machine used, and specially
to their working width, capacities range from 2 to 15 hours per
hectare.

Such machines are being
increasingly used in some tropical countries. In the Senegal
river delta region, private contractors or farmers' organizations
have recently acquired combine harvesters, mainly of the Western
type (Massey Ferguson, Laverda, etc.). So, almost 40% of the
Delta surface area is harvested with a pool of about 50 units.
Between 200 and 300 hectares of winter rice are mechanically
harvested. In this region the popularity of combine harvesters is
high despite their poor suitability for some small-sized fields.

In Brazil, several manufacturers
have adapted machines to rice growing conditions by substituting
tracks for wheels; some machines are simple mobile threshers
equipped with cutter bars.

In Thailand, local manufacturers
have recently transformed the IRRI thresher into a combine
harvester so as to reduce the labour requirement. The unit can
harvest 5ha per day and seems to have been rapidly adopted.

(iv) Strippers

Because of their size,
conventional harvesters and combine harvesters prove unsuitable
for many rice growing areas with small family farm holdings. In
response to this problem, research services, during the last ten
years, have developed small-sized machines for harvesting the
panicles without cutting the straw. Such machines are known as
strippers.

In the UK, the Silsoe Research
Institute (SRI) has developed a rotor equipped with special teeth
for strip-harvesting spikes or panicles. IRRI recently adopted
this technology and has developed a 10hp self-propelled 'stripper
gatherer' with a capacity of about 0.1ha per hour. However, the
harvested grain has to be threshed and cleaned in a separate
thresher. Since harvesting unthreshed produce results in frequent
stoppages for emptying the machine, this constitutes the main
drawback to the progress of the prototype.

In France, CIRAD-SAR has designed
and developed a machine which strips panicles from the plants and
threshes them in only one pass (Figure 4.5). The stripper has
been specially designed for harvesting paddy rice on small plots.
The essential component is a wire looped in line with the
direction of movement of the machine, which is mounted on a
three-wheeled carriage and powered by a 9hp engine. With a 30 cm
working width the stripper capacity is about 1 ha per day.

Maize Harvesting and Threshing

(i) Harvesting methods

Manual harvesting

In village farming systems the
crop is often harvested by hand, and cobs are stored in
traditional structures. Quite often, the crop is left standing in
the field long after the cobs have matured, so that the cobs may
lose moisture and store more safely after harvest.

During this period the crop can
suffer infestation by moulds and insects and be attacked by birds
and rodents. To reduce such risks, an old practice (called
"el doblado") is sometimes applied in South and Central
America. This involves hand-bending the ears in the standing crop
without removing them from the stalks. It helps mainly to prevent
rainwater from entering the cobs, and also limits bird attacks;
but, because of the high labour requirements involved, the
practice is gradually falling into disuse.

Manual harvesting of maize does
not require any specific tool; it simply involves removing the
cob from the standing stalk. The work time averages 25 to 30 days
per ha. Traditionally, maize cobs are commonly stored in their
unhusked form. To improve their drying, it is often recommended
to remove the husks from the cobs. Maize husking is usually a
manual task carried out by groups of women. Some machine
manufacturers (e.g. Bourgoin in France) have developed stationary
maize huskers, such as the "Tonga" unit.

Mechanized harvesting

The first mechanized harvester to
detach ears of maize from the standing stalks, the 'corn
snapper', was built in North America in the middle of the 19th
century. This was followed by the development of 'corn pickers',
which incorporated a mechanism for removing the husks from the
harvested ears. The first animal-drawn maize pickers were
replaced by tractor-drawn units (l or 2 rows) and then
tractor-mounted units (1 row). Finally came the development of
self-propelled units capable of harvesting from up to 4 rows. A
specific feature in maize harvesters is the header which leaves
the stalks standing as it removes the ears.

The rates of work can vary from 2
hours per hectare with a 3-row self-propelled harvester to 5
hours per hectare with a tractor-drawn or -mounted single row
unit. Generally speaking, harvest losses range from 3% to 5%, but
they may be up to 10%-15% under adverse conditions. Depending on
the situation, a single-row harvester can be employed effectively
on up to 20 hectares or more; but the use of a multi-row machine
demands several tens of hectares to be economically effective.

Specially designed for harvesting
maize as grain, the corn-sheller was initially a cornhusker in
which the husking mechanism was replaced by a threshing one
(usually of the axial type). Corn-shellers are self-propelled
machines of the 3 to 6-row type with capacities of 1 to 2 hours
per hectare (Figure 4.6. Maize sheller.). The surface areas harvested during a
180-hour campaign range between 100ha (with a 3-row unit) and
200ha (with a 6-row one).

Another alternative consists of
equipping a conventional combine with a number of headers
corresponding to the machine horsepower. However, although widely
used, such a method requires many adjustments to the threshing
and cleaning mechanisms.

(ii) Threshing methods

Shelling and threshing

Traditional maize shelling is
carried out as a manual operation: maize kernels are separated
from the cob by pressing on the grains with the thumbs. According
to the operator's ability the work rate is about 10kg per hour.
Outputs up to 20kg per hour can be achieved with hand-held tools
(wooden or slotted metal cylinders). To increase output, small
disk shellers such as those marketed by many manufacturers can be
recommended (Figure 4.7. Maize hand shellers.). These are hand-driven or powered
machines which commonly require 2 operators to obtain 150kg to
300kg per hour. Another threshing method, sometimes applied in
tropical countries, involves putting cobs in bags and beating
them with sticks; outputs achieved prove attractive but bags
deteriorate rapidly.

Motorized threshing

Nowadays many small maize
shellers, equipped with a rotating cylinder of the peg or bar
type, are available on the market. Their output ranges between
500 and 2000kg per hour, and they may be driven from a tractor
power take off or have their own engine; power requirements vary
between 5 and 15hp according to the equipment involved. For
instance the French Bourgoin "Bamba" model (Figure 4.8.
"Bamba" motorized maize sheller.) seems well-suited to rural areas in
developing countries because of its simple design, easy handling
and versatility (maize, millet sorghum, etc.).

Millet and Sorghum Harvesting and Threshing

(i) Manual harvesting

In Africa, and especially in the
Sudano-Sahelian area, these cereals constitute the staple food in
the human diet. They are harvested almost exclusively by hand,
with a knife (Figure 4.9. Knife ("ngobane")
for harvesting millet.)
after unroofing or bending the taller stems to reach the spikes.
Harvesting and removal from the field takes 10 to 20 days per
hectare, according to yields. Harvested ears are stored in
traditional granaries while the straw is used as feed for cattle
or for other purposes (e.g. thatching).

(ii) Gradual mechanization of threshing

Women separate the grain from the
ears with a mortar and pestle, as it is needed for consumption or
for marketing purpose (Figure 4.10). The threshed grain is
cleaned by tossing it in the air using gourds or shallow baskets.

This traditional method is
arduous and slow (10kg per woman-day). Consequently, research has
been conducted for some years on how to mechanize it.

The mechanical threshing of
sorghum ears does not raise any special problems: conventional
grain threshers can be used with some modifications; such as
adjustment of the cylinder speed, size of the slots in the
cleaning screens, etc. On the other hand, the dense arrangement
of spikelets on the rachis and the shape of millet ears
(especially pearl millet), make their mechanical threshing
excessively difficult.

The first millet and sorghum
threshers were developed in Senegal in the 1960-70s: the Siscoma
BS 1000 and the Marot DAK II. Giving relatively high outputs
(about 1000kg per hour) they have been intended for village
farmers' groups, cooperatives or private contractors going from
village to village to work on big threshing layouts. The
multipurpose "Bamba" thresher, better suited to rural
communities, has a capacity of about 300kg per hour. The

Senegalese pool of millet and
sorghum threshers currently amounts to 120-150 units.

As regards mechanized harvesting
at family level, some hand-operated threshers (Champenois) were
developed and tested experimentally but they did not prove very
successful. CIRAD is currently working on the design of powered
millet threshers of low capacities (50 to 100kg per hour).

Grain Cleaning

Threshing operations leave all
kinds of trash mixed with the grain; they comprise both vegetable
(e.g. foreign seeds or kernels, chaff, stalk, empty grains, etc.)
and mineral materials (e.g. earth, stones, sand, metal particles,
etc.), and can adversely affect subsequent storage and processing
conditions. The cleaning operation aims at removing as much trash
as possible from the threshed grain.

The simplest traditional cleaning
method is winnowing, which uses the wind to remove light elements
from the grain (Figure 4. 11).

(i) Mechanized cleaning

The most rustic equipment is the
winnower (Figure
4.12. Cereals winnower.):
a fan-originated current of air passes through several superposed
reciprocating sieves or screens. This type of machine was widely
used in the past for on-farm cleaning of seed in Europe. It can
be either manually powered or motorised; capacities range from a
few hundred kilogrammes to several tonnes per hour.

In Europe, with the use of
combine harvesters and the development of centralized gathering,
cereal winnowers have been progressively replaced by seed
cleaners in the big storage centres. These machines, also
equipped with a system of vibrating sieves, are generally capable
of very high outputs (several tens of tonnes per hour).

In developing countries,
mechanizing the cleaning operation at village level has seldom
been felt as a necessity, because of the lack of quality
standards in grain trading. However, because of the current trend
towards privatization of marketing networks, the demand for
cleaning machines will probably increase. The local manufacture
and popularization of simple and easily portable equipment, such
as winnowers or screen graders suited to cereal crops, need to be
encouraged. CIRAD/SAR has recently developed cleaning machines of
the rotary type with outputs of a few hundred kilogrammes per
hour.